In this May 28, 2019, file photo, a homemade sign is posted on a telephone pole in protest of Central Maine Power's controversial hydropower transmission corridor in Jackman. Credit: Robert F. Bukaty / AP

Supporters of Question 1 look to have the upper hand going into Election Day as a new poll found roughly half of Maine voters plan to vote yes to block the construction of the Central Maine Power corridor with another 15 percent still undecided.

The survey released by Digital Research, Inc. on Thursday is a positive sign for opponents of the 145-mile transmission line amid a hard-fought and expensive race, but the large share of undecided voters suggests some uncertainty with just five days before the election.

Mainers will vote Tuesday on the ballot question aiming to stop the project, which would carry hydropower from Quebec to connect with New England’s energy grid in Lewiston. The question would ban all transmission lines in the Upper Kennebec Region and require legislative approval for similar projects, including retroactively for the corridor. A yes vote would oppose the corridor, while a no vote would allow construction on the project to continue.

In the Digital Research Insights survey of 604 Maine voters during the month of October, 49 percent said they planned to vote yes on Question 1, 36 percent planned to vote no and 15 percent of voters declined to answer or were still undecided. The margin of error was 4 percent.

The poll did not find significant variation between voters in different regions of the state. But Democrats were more likely to support the yes side than Republicans and unaffiliated voters, it found. Some 57 percent of Democrats plan to vote yes in a break with Gov. Janet Mills, while Republicans and all other voters were almost evenly split in the low 40s between the sides.

Voting plans on the referendum were mostly aligned with overall views on the corridor. Sixty-one percent of respondents who had heard of the corridor said they opposed it, while only 29 percent supported it. Corridor opponents generally felt more strongly than supporters, with 39 percent saying they strongly opposed it compared to 14 percent strongly supporting it.

However, 12 percent of respondents said they opposed the construction of the corridor but still planned to vote no on the referendum.

Skepticism about the project’s benefits for Maine may be one factor driving opposition, the poll found. Only 9 percent of voters said they believed the corridor would have a positive impact on CMP customers, while only 20 percent said they thought it would be good for jobs in Maine. Just over a third of voters said they agreed with a statement often circulated by CMP allies that the corridor would create 1,600 permanent jobs here.

On the flip side, 52 percent of respondents said they thought the corridor would have a negative impact on Maine’s environment, although 60 percent said they believed the U.S. should prioritize or favor renewable energy sources over fossil fuels in the coming decade.

The extent to which hydropower is a clean source has been a flashpoint between proponents and opponents of the corridor over the past few months, with opponents of the project arguing wind and solar power are better alternatives, while supporters say the corridor would help phase out natural gas more quickly and complement other renewables later on.

Despite the high-profile nature of the referendum and the more than $87 million in political spending that has flowed into Maine in the past two years, public polling on the issue has been fairly limited so far. A Spectrum News Maine/Ipsos poll in September found relatively similar results, with 50 percent of voters opposed to the project and 34 percent in favor.

View a full list of the questions and cross-tabs based on party affiliation here and based on other characteristics here.